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First Position of Target.cpp
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First Position of Target.cpp
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/*
For a given sorted array (ascending order) and a target number, find the first index of this number in O(log n) time complexity.
If the target number does not exist in the array, return -1.
Link: http://www.lintcode.com/en/problem/first-position-of-target/
Example: If the array is [1, 2, 3, 3, 4, 5, 10], for given target 3, return 2.
Solution: Binary search
Source: http://www.jiuzhang.com/solutions/first-position-of-target/
*/
class Solution {
public:
/**
* @param nums: The integer array.
* @param target: Target number to find.
* @return: The first position of target. Position starts from 0.
*/
int binarySearch(vector<int> &array, int target) {
// write your code here
if (array.empty()) {
return -1;
}
int start = 0;
int end = array.size() - 1;
while (start + 1 < end) {
int mid = start + (end - start) / 2;
if (array[mid] < target) {
start = mid;
} else {
end = mid;
}
}
if (array[start] == target) {
return start;
}
if (array[end] == target) {
return end;
}
return -1;
}
};